In recent years, there is an increasing use of real-time applications, such as VoIP (Voice over Internet Protocol), VoPN (Voice over Packet Network), and streaming distribution, which perform transmission and reception of voice and image data between devices through communication networks such as an IP network. When a real-time application is used, the reproduction quality in reproducing voice and image data received on the application is influenced by the state of the IP network. Therefore, a guideline determined by the Ministry of Internal Affairs and Communications defines that providers of IP phone services should maintain certain communication quality for their services.
However, since a VoIP network including a plurality of communication networks such as an IP network and a public network, which provides IP phone services, is generally built in a multi-vender environment, it is difficult to secure and evaluate the transmission quality over the VoIP network. It is therefore necessary to provide an evaluation method for predicting communication quality between communicating terminal devices making a call, namely end-to-end communication quality, by simulating the characteristics of the communication network as the communication state over the VoIP network, and actually making a call between the terminal devices.
In a currently used evaluation method, since the characteristics of the communication network, such as the packet loss rate and delay on the communication network, are expressed as an average value, a maximum value, a minimum value, a standard deviation, or variance, etc., there is not sufficient information to simulate the characteristics of the communication network. In particular, there is a problem that it is not possible to accurately simulate a communication state including a change in the delay time of each packet and changes in the communication network characteristics such as burst performance with time, which is important to evaluate the communication quality of a VoIP call.
Then, for example, Patent Document 1 proposes a method for simulating a communication state including a change in the delay time of packets and burst performance. FIG. 17 is an explanatory view schematically showing the delay in the arrival time of packets. FIG. 17 schematically illustrates the results of measuring the communication state of packets as an explanatory view showing the relation between an expected arrival time and an actual arrival time. On the upper side of FIG. 17, the expected arrival times of the respective packets are illustrated in a time series so that time passes from left to right, and it is shown by five rectangles that packets assigned with the transmission order of the first to fifth are expected to arrive at transmission intervals d. On the lower side of FIG. 17, the actually measured arrival times of the respective packets are illustrated. As shown in FIG. 17, a packet transmitted first arrived with a delay time of 0d or as expected, but a packet transmitted second has a delay time of 3d. Similarly, packets transmitted third, fourth and fifth have delay times of 2d, 1d, and 1d, respectively. Specifically, in the VoIP network shown in FIG. 17, since one delay time 0d, two delay times 1d, one delay time 2d and one delay time 3 were measured, it is considered that the delay times 0d, 1d, 2d and 3d are distributed by 20%, 40%, 20%, and 20%, respectively.
FIG. 18 is an explanatory view showing schematically the results of conducting simulation tests about the communication state of a VoIP network, based on the measurement results of FIG. 17. The upper side of FIG. 18 shows times at which packets are to be transmitted from a transmission terminal device that transmits packets in a time series, and indicates by five rectangles that the transmission at transmission intervals d is set as the original transmission timings for the first to fifth packets in the transmission order. The lower side of FIG. 18 shows the results of simulating a VoIP network by adding delay times based on the measurement results shown in FIG. 17. As shown in FIG. 18, the delay time 1d is added to a packet to be transmitted first, and the delay times 2d, 3d, 0d, and 2d are added to packets to be transmitted second, third, fourth and fifth, respectively. As clear from FIG. 18, although a distribution of delay times is simulated, there is a reversal of transmission timing between the second packet and the third packet and also between the fourth packet and the fifth packet, and thus it is hard to say that an actual communication state is simulated.
FIG. 19 is an explanatory view showing schematically the results of conducting simulation tests about the communication state of the VoIP network, based on the measurement results of FIG. 17. The upper side of FIG. 19 shows times at which packets are to be transmitted from the transmission terminal device that transmits packets in a time series, and indicates by five rectangles that the transmission at transmission intervals d is set as the original transmission timings for the first to fifth packets in the transmission order. The lower side of FIG. 19 shows the results of adjusting the transmission timings to avoid the reversal of transmission timing, based on the results shown in FIG. 18. Specifically, when the transmission timing of the (n+1)th packet comes earlier than the transmission timing of the nth packet, an adjustment is made so that the (n+1)th packet is transmitted immediately after the nth packet. In the example shown in FIG. 19, the transmission timing of the third packet which comes earlier than the transmission of the second packet is adjusted, and the transmission timing of the fifth packet which comes earlier than the transmission timing of the fourth packet is adjusted. In this manner, a reversal of transmission timing is avoided. However, in the finally observed delay times, the delay time 1d was added to the packet to be transmitted first, and the delay times 3d, 2d, 2d, and 1d were added to the packets to be transmitted second, third, fourth, and fifth, respectively. This result shows a distribution different from the distribution of delay times representing the measurement results of FIG. 17, and it is also hard to say in this case that an actual communication state is simulated.    [Patent Document 1] Japanese Patent No. 2997607